The present invention relates to helicopter rotor systems. More particularly, the invention relates to a yoke for connecting a helicopter's rotor blades to the helicopter's rotor hub.
Yokes constructed of composite materials are known in the art. Examples of such yokes are described in U.S. Pat. No. 4,427,340 (Metzger et al.) and U.S. Pat. No. 4,650,401 (Yao et al.).
Each of the cited patents discloses a yoke that is constructed of conventional fiberglass materials in a polymer matrix. In each, a majority of the yoke structure comprises a unidirectional material in which all fibers are oriented in a spanwise, or longitudinal direction. This provides a structure with very high longitudinal strength to transmit high centrifugal forces from the rotor blades to the rotor mast, where the centrifugal forces balance each other.
Each yoke includes an arm for each rotor blade, and each arm includes a flapping flexure and feathering flexure. The flapping flexure accommodates most of the bending which results from rotor blade motion perpendicular to the rotor blade's plane of rotation ("flapping" motion). The feathering flexure accommodates most of the bending which results from rotor blade motion in the rotor blade's plane of rotation ("lead-lag" motion).
Control movements are transmitted to each rotor blade by a cuff. The cuff acts to rotate the rotor blade about its longitudinal, or spanwise axis, thereby changing its pitch, or "feathering" it. Rotor blade torsion loads are transmitted to the control system through the cuff.
The cuff's outboard end is rigidly attached to the inboard end of the rotor blade and its inboard end is connected to the yoke by a pair of elastomeric lead-lag dampers and an elastomeric shear restraint, or snubber. As explained in connection with FIG. 13 of Metzger, the cuff is more rigid than the feathering flexure in the lead-lag direction. As a result, when the feathering flexure is bent due to blade lead-lag motion, the lead-lag dampers are linearly deformed, thereby damping the blade's lead-lag motion. Such damping is necessary to prevent rotor blade instability due to ground and air resonance.
The feathering flexure's requirements are the most complex and difficult to fulfill:
1. It must be extremely strong in the spanwise direction to react rotor blade centrifugal force; PA1 2. It must be sufficiently rigid in the flapping direction to transmit flapping bending moments to the flapping flexure while not itself being bent significantly; PA1 3. It must have less lead-lag rigidity than the cuff so that lead-lag bending causes relative motion between itself and the cuff, thereby deforming, or "working" the lead-lag dampers, but it must have sufficient lead-lag rigidity to ensure that the deformation of the dampers is substantially linear, not rotary; PA1 4. It must have sufficient lead-lag shear strength to react the shear stresses which result from the relative motion between itself and the cuff; PA1 5. It should be torsionally flexible to minimize rotor blade control force;. PA1 6. It must have sufficient torsional shear strength to react the shear stresses which result from twisting it to feather the rotor blade; PA1 7. Its lead-lag rigidity must be such that the natural frequency of the yoke-cuff-rotor blade combination is about 65-70 percent of the normal operating RPM of the combination; and PA1 8. It should be relatively easy to fabricate using conventional composite layup and curing processes.
A feathering flexure's success in fulfilling the foregoing requirements depends largely on the geometry of its transverse cross section and the mechanical properties of the materials used in its construction.
The feathering flexure of the Metzger yoke develops relatively high shear stresses when twisted and is difficult to fabricate. Because of the relatively high shear stresses, the feathering flexure must be relatively long in order to provide the necessary pitch range without exceeding its shear strength.
The high shear stresses result from bending the feathering flexure's ribs 40, 42, 44 and 46 (Metzger FIG. 5) edgewise (parallel to their longer cross-sectional dimension) when the flexure is twisted. In addition, the intersections of the ribs 40, 42, 44, and 46 with the web 112 develop high intersection shear stress (which is proportional to the largest circle that can be inscribed in an intersection), because the ribs 40, 42, 44, and 46 are relatively thick at the intersections. Finally, the sharp corners at the intersections act as stress raisers, decreasing the shear strength of the flexure.
The shape of the Metzger feathering flexure renders it difficult to fabricate using conventional processes. This is due to the height of the ribs 40, 42, 44, 46 and the fact that they are perpendicular to the web 112.
In the feathering flexure disclosed in Yao, all shear stress is reacted by the flexure's spanwise filaments 56 (Yao FIG. 3b). While such unidirectional material is very efficient in reacting tensile stress, it is relatively inefficient in reacting shear stress. For example, the shear strength of unidirectional fiberglass material is approximately 10,000 psi, while its tensile strength is approximately 200,000 psi, a ratio of 1 to 20. As the ratio of lead-lag shear stress to spanwise tensile stress typically exceeds 1 to 20, the amount of material used in the flexure's construction is dictated by its required shear strength, which results in more tensile strength than necessary. A flexure constructed to achieve adequate tensile strength and shear strength simultaneously would save the material weight "wasted" in providing the excess tensile strength of the Yao feathering flexure.
Twisting the Yao feathering flexure to feather its rotor blade bends the arms of the flexure flatwise (parallel to their shorter cross-sectional dimension), which develops shear stress. For a given flexure arm thickness (the flatwise dimension), the level of shear stress developed is dependent on the amount the flexure is twisted per unit of spanwise length; for a given pitch change, the longer the flexure, the lower the shear stress. Thus, for a given pitch range, a feathering flexure having higher shear strength than the Yao flexure could be shorter than the Yao flexure. Among other advantages, a shorter flexure would save weight and reduce aerodynamic drag.